Single phase induction motors are widely used, due to their simplicity, strength and high performance. They are used in household appliances, such as refrigerators, freezers, air conditioners, hermetic compressors, washing machines, pumps, fans, as well as in some industrial applications.
These single phase induction motors are normally provided with a cage type rotor and a coiled stator having two windings, one being for the running coil and the other for the starting coil.
During normal operation, the running coil is supplied with an alternating voltage and the starting coil is temporarily supplied at the beginning of the operation, creating a turning magnetic field in the air gap of the stator, a necessary condition to accelerate the rotor and promote starting.
This turning magnetic field may be obtained by supplying the starting coil with a current that is time-displaced in relation to the current flowing in the running coil, preferably at an angle close to 90 degrees.
This time displacement between the currents flowing in both coils is achieved by constructive characteristics of the coils, or by installing an external impedance in series with one of the coils, but typically in series with the starting coil. Typically, the value of the current flowing in the starting coil during the starting operation of the motor is high, being necessary to use a switch to interrupt this current after the time required for promoting acceleration of the motor has elapsed.
In motors in which high efficiency is required, said starting coil is not completely disconnected upon completion of the starting period. A capacitor, namely a running capacitor, is kept in series with said starting coil, providing enough current to increase the maximum torque of the motor and the efficiency thereof.
For a motor having such configuration, employing a permanent impedance in series with the starting coil during the normal operation of the motor, some starting devices of the PTC or electronic types are known, as described in U.S. Pat. No. 5,051,681.
The known prior art starting circuits using a PTC as a starting device have some inconveniences, such as high energy consumption, as described in Brazilian document PI201210.
As described in U.S. Pat. No. 5,051,681, the starting circuits having an electronic starting device usually employing a triac, although not having problems of energy consumption like the circuits using PTC, have the inconvenience of being susceptible to voltage variations and, with the occurrence of voltage transients or determined conditions in which power supply to the motor is cut off, conducting the circuit to restart the motor, regardless of the powering condition in which said motor is still found, allowing the occurrence of current overload in the starting circuit, resulting in burns of determined components thereof due to overheating.